1. Field of the Invention
This invention relates generally to light direction control, and more particularly, to techniques for the dynamic control of light propagation direction.
2. The Prior Art
The dynamic control of light propagation direction is a fundamental technique in optics. Direct applications include laser radar systems, laser projection displays, target-tracking systems, land survey systems, entertainment, laser printers, laser machining, metrology, laser scanning, and optical communications.
Currently, there are four significant light deflection methods: electromechanical, acousto-optic, electro-optic, and electrically-controlled light grating methods. Of these four methods, electromechanical methods are used most frequently in commercial application products. Electromechanical methods use a rotating mirror or rotating prism as a mechanical device for changing light direction. These devices have a number of limitations due to the intrinsic nature of mechanical movement on a macroscopic scale. For example, they are relatively slow. Generally, it takes milliseconds for changing the light beam from one direction to another direction. Also, such systems are susceptible to interference from mechanical vibration.
The other three types of methods contain no mechanical moving parts at the macroscopic scale. However, the maximum deflection angle range often constitutes an important limiting factor to their performance. For example, the maximum deflection angle that the fully electronic control methods of the prior art can provide is generally less than .+-.3.degree.. The small deflection angle essentially excludes electronic control methods from nearly all important practical applications. Electromechanically-controlled rotating mirror devices can provide moderately larger deflection angle. The maximum deflection angle for two-dimensional electromechanically-controlled rotating mirrors is usually much less than .+-.30.degree., limited by the geometry of mechanical parts. And, in many important light scanning applications, such as laser radar systems, a much larger scanning angle range is often required. Thus, even the maximum deflection angle range of the electromechanical systems is still insufficient.
In the prior art, U.S. Pat. No. 4,836,629, issued to Huignard, discloses a holographic multiplexer system for changing light beam direction in a broad field of view. The key component is a holographic multiplexer, the operation of which is based on the wave nature of light. Specifically, in the Huignard system, a light beam is split into multiple beams with different directions of propagation through the interference of coherent electromagnetic waves. Huignard then uses a shutter to select which of the light beams to output from the system. One shortcoming of the Huignard system is that the energy of the output light is only a small percentage of the input light energy. For example, if the holographic multiplex outputs a matrix of N.times.N=N.sup.2 beams, each beam will have a maximum energy level of only 1/N.sup.2, and since N.sup.2 is typically in the hundreds or thousands, the output energy is very small indeed when compared to the input energy.
It must be noted that the holographic hardware and process are often too complicated for practical applications. For example, Huignard's system uses an additional laser system, a two-dimensional phase modulator, and additional nonlinear optical material for pumping external laser energy back into the output light beam in order to compensate for the light energy lost in the holographic multiplexing process. Quantitatively, the additional laser must have a power output many times higher than that of the incident laser source in order to compensate for losses in the pumping system. Additionally, for the multiplexer to work, a rather delicate 2D detector system must be used to first record a hologram and then use a rather complex process to reconstruct the original wave front for providing multiple split beams. This all makes the Huignard system too complex and delicate for many practical applications.
In terms of classical geometry optics, the standard method for increasing an initially-deflected light beam deflection angle a small amount (typically .+-.5.degree.) uses a light beam expander. As shown in FIG. 2, the beam expander is a lens system having two positive lenses sharing a common focal plane 112. The focal length f.sub.1 of one lens 102 is much larger than the focal length f.sub.2 of the second lens 104. When a well-collimated light beam travels from the second lens 104 to the first lens 102, as at 106, the beam size is increased by a factor of f.sub.1 /f.sub.2. When a beam expander is used in the reverse direction, that is, from the first lens 102 to the second lens 104, as at 108, the beam size is reduced and the deflection angle of the input beam relative to the optical axis 110 is enlarged by a factor of f.sub.1 /f.sub.2. When f.sub.1 /f.sub.2 is large, however, the most important limiting condition is the maximum deflection angle of the output light beam. The output light beam always goes from the output surface 114 of the small lens 104 to the optical axis 110, and crosses the optical axis 110 at a distance .delta. from the small lens 104. The ratio of r/.delta., where r is the radius of the small lens 104, directly determines the maximum deflection angle.
A telescope eyepiece has an even more specifically defined purpose. Because the user looks directly into the system with the eye, the light beam output is designed to be about 5 to 10 mm away from the surface of the small lens, and the light beam is about 2 mm in width to match the pupil size of a human's eye.
It can be seen, therefore, that the beam expander or a standard telescope eyepiece is a device that largely provides an output light beam in the near field. The maximum deflection angle range is very limited because the output light beam always goes towards the optical axis and the value of .delta. cannot be very small. Also, the light beam distortion and degradation is large, because the output light beam comes from the edge of the lens. When a compound lens system is used, these lens system are designed to function at the near field of the lens system, not the far field. After the light beam passes through the optical axis and into the far field, the beam degradation often becomes unacceptable.